Are exercises with or without occlusal splints more effective in the reduction of pain in patients with temporomandibular disorders of myogenic origin? A systematic review

Abstract Temporomandibular disorders (TMD) is a term used to describe a set of clinical conditions that may compromise the temporomandibular joint (TMJ) and masticatory muscles and/or associated structures, considered the most frequent cause of orofacial pain of non-dental origin. In recent years, many forms of physical therapy have been used in the treatment of TMD to reduce pain and improve the range of mandibular movement present in this impairment. Among these resources are kinesiotherapy (exercise), electrothermal and manual therapy, acupuncture, training posture, mobilizations, and biofeedback. Objectives To determine if exercises with or without occlusal splints are effective in reducing pain in patients with temporomandibular disorders (TMD) of myogenic origin. Methodology This systematic review was registered in the International Prospective Register of Systematic Reviews (CRD 42019134244). Controlled trials published in PubMed, Scopus, and Cochrane Library following PRISMA guidelines up to April 2022 were randomized and included. The population above 18 years, which evaluated the effectiveness of exercise with or without occlusal splints in reducing pain in patients with TMD of myogenic origin, diagnosed through the Research Diagnostic Criteria for Temporomandibular Disorders, was also included. There was no restriction on the period of publication. Cochrane risk of bias analysis was performed. Results Of the five included articles, all showed a reduction of pain, but without significant differences between the interventions performed. Additionally, studies that evaluated the quality of life and mandibular movements showed a reduction in pain, but no significant differences between therapies. Conclusion The analyzed studies showed no difference in the improvement of pain, quality of life, and mandibular movements between the groups that performed only exercises or the associated treatments.


Introduction
Temporomandibular disorders (TMD) is a term used to describe a set of clinical conditions that may compromise the temporomandibular joint (TMJ) and masticatory muscles and/or associated structures, 1-3 considered the most frequent cause of orofacial pain of non-dental origin. 3,4 The signs and symptoms of TMD are sensitivity to palpation of the masticatory muscles, pain, restrictions of joint movement, noise in the temporomandibular joint (TMJ) headache, and tinnitus, among others. 4,[6][7][8][9] While TMD is uncommon in childhood, its prevalence increases in adolescence and young adulthood, affecting people mainly in the age range of 20 to 40 years. Females are the most affected, presenting greater symptom severity and experiencing more difficult recoverying. 7,8 Its etiology is multifactorial, involving genetic, individual, and environmental factors. Studies showed that increased stress is associated with the appearance of muscular tension, which affects the functioning of the stomatognathic system and TMJ. 8,10 TMD is a multifactorial pathogenesis, and its treatment must be interdisciplinary, combining a conservative approach, cognitive behavioral therapy, physical therapy, and pharmacology intraoral devices where applicable. 11 Occlusal splints (OS), which promotes the relaxation of the masticatory muscles, has been widely used aiming to restore neuromuscular balance. Moreover, it repositions the mandibular condyles and normalizes the proprioception of the periodontal ligament, providing a correct alignment of the temporomandibular joint. Such removable device is made of acrylic resin with polymer and can be used during the day or at night. 3,5,[13][14][15][16] In recent years, many forms of physical therapy have been used in the treatment of TMD to reduce pain and improve the range of mandibular movement present in this impairment. Although the literature lacks consensus on the best treatment for this disorder, some therapies such as kinesiotherapy (exercise), electrothermal and manual therapy, acupuncture, training posture, mobilizations, and biofeedback can be used in the management of TMD. 6,7,11,17 Orofacial exercises have been one of the main interventions in the management of TMD. Aiming to promote the proper functioning of the cranio-cervicomandibular region, strength, mobility, coordination, the relaxation of the jaw muscles, the increase of range of motion of the TMJ, proprioception, and muscle relaxation, this intervention brings positive effects on pain. The exercises must be part of an individualized program, in addition to the need for verbal and written instructions. 18,19,20 This study aimed to perform a systematic review of the literature to verify if exercises with or without the OS are effective in the reduction of pain in patients with TMD of myogenic origin. The hypothesis of the study is the physical therapy exercises associated with the occlusal splint are more effective in the treatment of pain in myofascial TMD.

Eligibility criteria
The problem, intervention/indicator, comparison, and outcome of interest (PICO) question guiding this study was the following: "Are exercises with or without the OS effective in reducing pain in patients with TMD of myogenic origin?" Here, "P" was "patients with TMD of myogenic origin"; "I" was "physical therapy exercises"; "C" was "use of the OS + physical therapy exercises"; and "O" was "pain assessment (primary) and quality of life and mandibular movements (secondary)."

Exclusion criteria
Studies that used other tools for TMD diagnosis, assessed TMD unrelated to muscle origin, or included patients with diseases that could affect masticatory muscles or the TMJ, were excluded.

Study selection
The selection process was conducted in two phases.
In the first phase, two independent evaluators (JFOLB and JMLG) read the titles and summaries of the studies identified in the databases searched. Studies that did not meet the inclusion criteria were discarded. In the second phase, the same researchers applied the eligibility criteria to the full text of the articles. During the searches, disagreements were resolved by a third reviewer (MVH).
Inter-examiner evaluation (kappa test) was performed to verify the level of agreement between the

Data collection
One author (JFOLB) collected the relevant data from the articles, which were then verified by two other authors (JMLG and MVH). The qualitative data collected included author/year/country, study design, follow-up, age group, intervention (types of exercises, habit counseling, and occlusal splints), assessment instrument for pain outcome, results (pain, quality of life, and mandibular movements), conclusion, and effect.

Risk of bias
The bias risk analysis was performed by two independent authors (BCEV and SLDM) using the Cochrane risk of bias tool. For each manuscript, the items' allocation confidentiality, the possibility of randomization, blinding, and patient loss were examined. Finally, the studies were categorized according to the level of risk of bias as "low," "medium," or "high".

Selection of studies
The database search yielded a total of 1.111 studies in the databases as follows: 374 Pubmed/ MEDLINE, 166 Cochrane Library, and 571 SCOPUS.
After the exclusion of duplicated studies (368), 743 studies were evaluated through the reading of titles and abstracts. Among the 31 articles remaining for full-text evaluation, after applying the eligibility criteria, 27 were excluded for several reasons. Manually searching, we found one article that met the inclusion criteria.
Finally, five articles were part of the data extraction and qualitative synthesis. Figure 2 shows the process of identification, inclusion, and exclusion of studies.
The inter-examiner test (kappa) showed "moderate" to "almost perfect" agreement according to the criteria (K=0.88) for each database.
Characteristics of the studies Figure 3 summarizes the articles, highlighting the authors, year of publication, country of origin, age group, sample, follow-up, intervention, assessment instrument for pain outcome, results, and conclusion.
As for the temporal distribution, studies were conducted in 2012, 22 2013, 21 2015, 22 2017, 27 and 2018. 24 Regarding geographic distribution, one study was conducted in Brazil, while the others were conducted in Finland, 20,24 the Netherlands, 27 and India. 21 The age range of the study populations was broad, covering both young and adults. 20,21,22,24,27 The follow-up period varied among the studies: one month, 20 five months, 22 six months, 21 and one year. 24 In one study, 27 the follow-up time, dependent on the rate of symptom improvement, differed for each patient.
In our review, we found that there was a reduction in pain between the initial assessment and after the  intervention, but no significant difference regarding whether the studies used the occlusal splints with exercises [20][21][22]24 or isolated therapies. 27 Only one study 24 evaluated the quality of life related to oral health (QLROH), observing an improvement in the dimensions of physical capacity and eating discomfort, regardless of the treatment used. Niemelä,et al. 22 (2012) and Katyayan, et al. 21 (2013) evaluated mandibular movements and reported that the splints treatment was not more effective than exercises for increased mandibular mobility.

Risk of bias
All included studies were classified as low risk of bias according to the Cochrane risk of bias tool, 27 although none of them met all the criteria for methodological quality since some information was unclear. Figure 4 shows a graph on the risk of bias and Figure 5 shows a more detailed analysis of each study. The SG showed improvement in mandibular movements. Pain on palpation increased in the GP while it decreased in the CG, but the difference in change of any of the variables between groups was not statistically significant (p = 0.05).
The study did not demonstrate that splint treatment in combination with exercise has a greater benefit in relieving facial pain and increasing joint mobility when compared to only exercise over a short period. There was a significant reduction in headache intensity for both groups between onset and after two and five-month followups (p < 0.001). There was no difference in the reduction of pain intensity between two and five months both within and across groups.

NONE
In patients with headache attributed to TMD, counseling was effective in improving pain, but the addition of the occlusal appliance did not offer an additional therapeutic effect. There was no difference between the groups regarding the quality of life related to oral health (QLROH during follow-up.

NONE
After one year, the facial pain and discomfort when feeding reduced.
There was improvement in the QLROH during one year of follow-up, regardless of the treatment performed, in terms of physical pain and psychological discomfort.   Regarding the protocol of splints use, patients were instructed to use it overnight, except for one study that advised that the splints should be used as long as possible or for at least 10-12 hours at night. 29 Deregibus, et al. 3 (2021), in their randomized clinical trial (RCT), guided participants to use it at night, for at least 8 hours, while Melo, et al. 16 (2020) stated that OS can be used in the morning or at night, depending on the patient's clinical condition, which must be analyzed by a specialist. We should note that the Finnish guidelines recommend that splints should be used at night because it is uncomfortable to use during the day owing to work activities, school, and feeding. 30 However, the literature lacks consensus about the optimal period and time of occlusal splints usage.
Regarding the exercises used, Kokkola, et al. 26 (2018), Niemelä, et al. 22 (2012 and Katyayan, et al. 23 (2013) implemented the protocol proposed by Carlsson and Magnusson 30 (1999). This program consists of mouth opening, lateralization, and protrusion, actively and with maximum maintenance of the positions for a few seconds; it is, essentially, exercises that incorporate resistance from the patient's fingers. These exercises are repeated from seven to ten times and performed two to three times per day. Costa, et al. 24 (2015) used stretching and elongation exercises three to five times per second. Already Van Grootel, et al. 25 (2017) performed the Jacobson method, which promoted the relaxation of the mandible muscles in addition to stretching, mandibular movement, and self-massage.
Fernández-de-las-Peñas and Piekartz 31 (2020) state that mandibular exercises should be individualized aiming to improve muscle coordination, increase the range of motion, and relax strained muscles. There is no standardization of the exercises used to treat TMD, as to patient compliance with therapy, appropriate dosage, and exercise frequency. Despite this, it is possible to compare studies because of commonalities regarding relaxation, active movements, and stretching.
A common point in all studies [22][23][24][25][26] was the use of guidelines, advice, and instructions on the etiology and prognosis of the disease, diet, and parafunctional habits to avoid. Costa, et al. 24 (2015) stressed the importance of sleep hygiene and the application of hot cushions on painful muscles. They also stated that counseling and behavioral changes were effective in relieving headaches attributed to TMD because the use of the occlusal splints did not offer any additional therapeutic effect. This reinforces the need for a more comprehensive protocol, involving professional intervention as well as counseling strategies and behavioral changes since these approaches are reported to be a valuable tool for the management of myofascial pain. 12 Studies observed that patients with masticatory muscular pain have alterations in respiratory patterns, depression, and sleep disorders.
Therefore, interventions that address bio-behavioral strategies, alone or in combination with other therapies, could help alleviate pain. 31,32 The studies combining OS and exercises, making a comparison with only exercises, [22][23][24]26 as well as the study comparing individual therapies, that is, only the OS compared with exercises, 25 concluded that the treatments were effective in improving pain.
However, no significant difference was observed between the treatment group and control groups. [22][23][24][25][26] We should highlight that OS and exercise therapy have similar success rates, 18 with their efficacy being confirmed through follow-ups. 28 Therefore, according to Van Grootel, et al. 25 (2017) the key to the choice of treatment is patient compliance and preference.
The impact of pain on the quality of life in patients with TMD was assessed only by Kokkola, et al. 26 (2018), who found that although the patients experienced an improvement in physical pain and discomfort when eating, no difference was observed between the interventions (OS and exercises vs exercises). However, TMD-related pain has a greater impact on QLROH than dental problems, and it is also known that patients with TMD suffer more, especially regarding the physical pain of TMD. 33,34 Mandibular movements were evaluated by Niemelä,

Conclusion
The analyzed studies showed no difference in the improvement of pain, quality of life, and mandibular movements between the groups that performed only exercises or the associated treatments. Due to the heterogeneity of the studies analyzed, further research is needed.